The present invention relates to a method and apparatus for earth oriented alignment of a satellite which is deployed in a relatively low earth orbit (substantially lower than the geostationary orbit), at a median inclination. "Median inclination" means that the three orthogonal components of the earth's magnetic field, expressed in an earth oriented moving orbit reference system, are generally nonzero.
Normally, the solar panels of a satellite are rotated and tilted toward the sun corresponding to the time of the day and year. Thus, relatively large deviation moments occur in the inertia sensor of the satellite which, in turn, results in very high disturbance torques because of the gravitation gradient.
Attitude control of a satellite in the above-described orbital path is normally carried out by means of reaction wheels and magnet coils. (Usually nozzles are used only for the spin discharge of the wheels.) Attitude control systems which operate only with wheels and magnet coils usually have three axis-related magnet coils. However, the technical publication by Fisher et al, "Magnetic Momentum Bias Attitude Control with Two Gimballed Appendages", 18th Annual AAS Guidance and Control Conference, Keystone, Colo., AAS Paper 95-005, discloses a process which requires only two magnetic coils, which are mounted along the x- and y-axes of the satellite.
The present invention also relates to a control arrangement which uses only two magnet coils. Control laws and strategies for disturbance compensation are discussed, however, which differ from those of Fisher et al, as well as an alternative arrangement of the magnet coils and the associated control processes, which achieve a clear improvement of the control action.
One object of the invention is to provide a process and apparatus for earth oriented control of a satellite, which requires only two magnet coils, a spin wheel and a two-axis measuring earth sensor.
In comparison to the prior art as exemplified by Fisher et al, the present invention differs in the following important respects:
i) The two magnet coils are preferably arranged along the satellite x- and z-axes. PA1 ii) No magnetometer is used. PA1 iii) For the satellite attitude and nutation control, not one, but both coils are used. PA1 iv) Control laws are used which are adapted to the special magnet coil configuration and exhibit significant improvements with respect to previous processes. PA1 v) Measures for disturbance compensation and the disturbance reduction are provided which also improve the control performance.
Points ii) to v) are advantageous not only in connection with a magnet coil configuration according to point i), but also in the case of the use of an x/y magnet coil configuration, as described by Fisher et al.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.